Rail and Maritime Transport Union general manager Wayne Butson said members were bearing the brunt of passenger complaints about no heating in trains during the past fortnight, which he believed was a health issue.

"Commuters are angry about being cold because of broken heating units. Our members do not like working on cold trains either.

"We know Wellington's rail fleet is under pressure but, considering we are not getting new rolling stock till 2010, we have to make sure existing stock is kept in good condition - especially in winter."

Christine Hofkens, who commutes from Paraparaumu to Wellington five days a week, described the carriages as "bloody cold" and said it caused problems with her arthritis.

She has resorted to the same thing she did last winter during similar heating failures - wearing extra layers of clothing she peels off once she reaches work and carrying a large blanket. "It is terrible, it is like living in poverty. The icy air affects the joints in my feet."

She said she had complained to KiwiRail and been told lighting was a higher priority than heating.

KiwiRail spokesman Nigel Parry said 12 of its 100 carriages had defective motor alternators, which powered heating and lighting. The problem reduced the amount of power available in the system. "We may not be able to have heating on all of the trains, but we can still have lights and doors."

Lighting and working doors were essential to safety, he said.

"People have to be seeing where they are going when they step on and off . . . We are working as hard as we can to fix them. Spare parts are not always readily available and, because of their age, we cannot rule out further problems."

Six carriages with electrical problems were being repaired, and KiwiRail was waiting for parts to fix the others.

Greater Wellington regional council plans to spend $210 million on more than 70 new trains due to be in use about the middle of next year.